Combination locks are generally mounted on the inside of a door and include a long rotatable spindle running through the thickness of the door. A graduated dial is fastened to the front end of the spindle at the outside face of the door. A lock case, fitted on the inside face of the door and containing the lock mechanism, accepts the other end of the spindle. Inside the lock, the spindle generally runs through a hollow cylinder. A plurality of circular tumblers are coaxially mounted for rotation on the hollow cylinder. The spindle terminates at a driver which is a disc smaller than the tumblers but larger than the cylinder. The driver is keyed to the tip of the spindle just beyond the end of the cylinder. On the side facing the tumblers, the driver carries a small stub which engages similar stubs on the tumblers. This enables positioning of the tumblers by the driver by rotation of the dial the required number of turns in either direction. Each tumbler also incorporates a gating, the gatings being aligned for the unlocked condition. In general, drivers also incorporate a gap at their edge. The locks include a bolt to which is pivoted a spring-loaded drop arm. The drop arm usually includes a hook-like member to engage the gap in the driver and a projecting bar positioned outside the edges of the tumblers for engaging the gatings in the tumblers.
In some forms of locks the projecting bar rests on the edges of the tumblers with the hook-like member being suspended above the edge of the driver. As the tumblers are rotated by the driver, it is possible to analyze the changes in resistance when the gatings pass beneath the projecting bar. Such locks are pickable in this manner and can be compromised.
In other types of conventional locks, the projecting bar lies suspended outside the edges of the tumblers while the hook-like member rests on the edge of the driver. Picking by analyzing frictional changes is not generally possible in these locks. However, as the tumblers are rotated by the driver, at one time during each rotation the gap in the driver passes underneath the hook-like member. In this position the hook-like member stays in a suspended position because the projecting bar rests on the edges of the tumblers. The gap in the driver is generally a substantial portion of the driver's overall circumference. Hence, the combination numbers coinciding with the gap in the driver are still pickable. On average, the gap covers about ten percent (10%) of the circumference of the driver. Thus in a four (4) tumbler lock, only about sixty-five percent (65%) of the total four (4) number combinations are truly unpickable; the remaining thirty-five percent (35%) of the combinations are still fully or partly pickable.
A further disadvantage of existing combination locks is that a person using the locks will neither be able to identify the two (2) categories nor differentiate between the pickable and unpickable ranges in locks of the second type. Hence, a user is always uncertain about the degree of security for all combinations in all locks.
Apart from pickability by analyzing frictional changes, all forms of the above-mentioned combination locks fail to offer adequate resistance to force attacks due to their use of a single direct drive spindle. A common method employed by criminals to compromise such locks is to shear, break or disjoin the dial on the outside and drive the spindle inwards. The spindle and the driver fastened thereto generally dislodge the cap of the lock case. In some situations, the entire lock or the part of the lock containing the bolt is dislodged from position; thus rendering the lock ineffective and facilitating opening of the safe. In other cases because the spindle hole passes through the central part of the lock, the lock can be easily dislodged through the cleared hole using suitable tools. Thus the direct drive spindle arrangement is a very serious weakness in the combination locks known in prior art.
Conventional combination locks of the type previously referred to have certain other inherent weaknesses which are equally serious. When a conventional combination lock is locked, it does not provide the user with any direct or distinct indication that all tumblers are effectively scattered and that it is at full security. Throughout the world the accepted method for locking is to rotate the dial at random. According to "An Encyclopedia of Locks and Builder's Hardware" published by Josiah Parkes and Sons, Ltd., a "few random turns" are required to "effectively scatter the combination" and bring the lock to "full security". There is however, no clear-cut instruction from lock manufacturers and dealers on the actual or minimum extent of what is "a few random turns".
In the absence of specific instructions, a user generally rotates the dial one (1) or two (2) turns both ways a few times and believing that he has effectively executed locking, leaves the dial in position. He is unaware of the real end result of the rotating movements, which based on various factors, particularly the combination numbers and direction and extent of rotation, can vary considerably in the same lock or from lock to lock.
In a four (4) tumbler combination lock as an example, in some cases a one (1) turn rotation can effectively scatter (or move from original unlocking positions) all four (4) tumblers; but in the same lock in some other cases, even two (2) rotations beginning from the unlocked, bolt withdrawn condition, may only scatter one (1) or a maximum of two (2) tumblers from the distal or driver end. This would leave the other three (3) or two (2) proximal end tumblers that are furthest from the driver in their originally set, unlocked position. In such cases, pick locks can easily find out the combination numbers for the unscattered proximal end tumblers even if such tumblers are not pickable by analyzing frictional changes. This is done by what is known as the continuous one-way rotation method. Using this method the pick-lock rotates the dial in a direction, either clockwise or anti-clockwise, from the position in which the lock has been left by the user. The rotation is continued over a required number of turns. By studying the intermittent and distinct increases in load, the pick-lock may discover the numbers at which the tumblers get connected one after the other for rotation. Even though the first one (1) or two (2) numbers related to the one (1) or two (2) scattered tumblers are of no use, the last two (2) or three (3) numbers related to the two (2) or three (3) unscattered tumblers would be the precise combination numbers for unlocking the lock, but in the reverse order.
The dial-spindle-driver assembly of conventional locks by itself offers hardly any resistance to rotation. The tumblers are usually lightly spring-loaded on purpose to provide some stability in resistance to rotation. For such arrangement the increases in rotational load as the stubs on the tumblers engage can be clearly felt and identified by a pick-lock.
In the event a lock is not precision made or properly assembled, every other number may vary by a unit or so from the originally set number due to the change in direction of rotation required thereat. However, the expert pick-lock can easily ascertain the correct direction, the quantum of variations, if any, and the numbers requiring adjustments to make the necessary correction. In addition, if the pick-lock gets one (1) or more opportunities to study the numbers prior to his final attack, he can compare and confirm the combination numbers. In cases of "inside jobs" or attacks with inside support, such opportunities are adequately available.
Armed with the combination numbers for the proximal end tumblers, it will not take much time for a pick-lock to unlock the lock even if the combination numbers for the scattered distal end tumblers are not pickable by analyzing frictional changes. By taking advantage of the weakness of conventional combination locks, a pick-lock can actually try every combination for the remaining tumbler or tumblers in a short time by selective manipulation. In this method the pick-lock can overcome the need for repeating complete cycles of operations when attempting various combinations. He simply keeps the proximal end tumblers furthest from the driver positioned according to the numbers he has previously determined using the continuous one-way rotation method. Thus, he can confine his work to the remaining tumbler or tumblers for which the combination numbers remain unknown. By selectively engaging only the remaining tumblers and by operating in the required sequence and direction, the pick-lock can complete the task within manageable time limits. Being an expert he would probably require a maximum of about five (5) minutes if only the last number is unknown and a maximum of about nine (9) hours if the last two ( 2) numbers are unknown. Because the pick-lock would not have to try all combinations for the unknown numbers before discovering the correct combination numbers, on average it would only take about one-half (1/2) the maximum times quoted.
Discontinuing the process of unlocking a conventional combination lock halfway through the process, presents another risk. A number of users thinking that no scattering is needed as they have not opened the lock, may leave their locks in such conditions. Others may leave the process of dialing the combination partially completed to minimize the time required to gain access to the safe or vault. Such persons generally seldom appreciate the risks involved.
A conventional combination lock left in the fully unlocked condition can similarly reveal all of its combination numbers to pick-locks. A dishonest assistant entrusted with the task of locking a combination lock can easily find out all the numbers for the combination in the process of his locking by applying the continuous one-way rotation method. Such situations occur often in dual control arrangements where a superior officer controlling one lock instructs his assistant to lock it at the end of the day.
Thus, there exists a need in the prior art for a lock with a simple, speedy and standard system for instant locking and for providing at all times and for all combinations the maximum security which can be achieved by fully scattering the tumblers. There further exists a need for a lock which overcomes the insufficiency of existing locks that lack any facility to indicate the degree of scattering of the tumblers when the lock is attempted to be secured. The fact that the tumblers may reveal their individual positions by load increases when the tumblers are left fully unscattered is a further disadvantage of prior art locks. Combination locks of conventional design further suffer from the deficiency that a pick-lock having learned the first several combination numbers may through repeated positioning of the tumblers closer to the driver, discover the remaining numbers without having to disturb the previously positioned tumblers further from the driver. The fact that combination data can be extracted by the continuous one-way rotation of tumblers in a lock which has been left in the open position is also a very serious weakness endangering the security of locks known in the prior art. All of these deficiencies establish the need for a lock having enhanced security features.
In addition to pickability by analyzing frictional changes and susceptibility to forced attacks and the other weaknesses and disadvantages mentioned above, conventional combination locks also cause some serious operating discomfort to their users. When a lock of conventional design must be unlocked, the tumbler positioning process of even a four (4) tumbler lock involves fourteen (14) closely observed rotations beginning in a prescribed direction and changing directions after five (5), four (4), and three (3) rotations. This apart from being time-consuming and irritating causes real confusion and chaos to many. In addition, if a conventional combination lock is to be reset to a new combination, the process is extremely laborious, involving three (3) extra cycles of tumbler positioning operations and requiring a total of forty-four (44) extra rotations.